1. Field of the Invention
The invention relates to a piston for combustion engines, with a lower part surrounding the shaft zone and with an upper part connected with the said lower part via expansion screws and forming the base and the ring zone, the expansion screws in the lower part resting on expansion sockets engaging respective borings in the lower part of the piston, while the supporting action between the expansion socket and the lower part of the piston is provided via a convex surface on the said expansion socket and a correspondingly concave surface on the lower part of the piston, the radius of the convex surface being smaller than that of the corresponding concave surface.
The expansion sockets are used for the purpose of imparting a certain prestressing to the screws, this being required in order to ensure a reliable connection between the upper and the lower part of the piston in operation.
2. Description of the Prior Art
In the constructions hitherto in use the central points of the radii of the convex and concave bearing surfaces were in each case both situated on the axis of the screw holes in the lower part of the piston. The radii are made different in the manner mentioned in order to ensure that when the screws are tightened up, in the outer zone of the bearing surface of the lower part of the piston, the expansion socket will not be overstrained. For it has been found in practice that in many cases this zone is in danger of developing cracks. Such overstraining was particularly liable to occur when the bearing surfaces on the expansion socket and lower part of the piston were still made flat. When the bearing surfaces are made flat it is impossible, for reasons connected with manufacturing accuracy, for an even supporting action to be obtained over the entire surface. On the contrary, it is certain which zones of the bearing surfaces will be subjected to extra strain. The adoption of convex-concave bearing surfaces, in which the radius of curvature for the expansion socket bearing action is smaller than the radius of curvature for the lower part of the piston, ensures that the expansion socket will first of all be supported in the inner edge zone of the boring, contact over the entire surface being deferred until the screw is tightened up. This prevents the outer zone of the bearing surface of the lower part, the zone exposed to the danger of cracking, from being subjected to an overload conducive to the formation of such cracks.
In this version, however, the internal zone of the boring, i.e. the inner edge of the bearing surface for the expansion socket, in the lower part of the piston, may in certain cases suffer excessive strain.